fix(grunt-utils): insert the core CSS styles without using innerHTML

Create style elements and modify their text content instead of using
innerHTML to create the whole `<style>` element with its content.
That way style insertion done at bootstrap time doesn't interfere with
Trusted Types restrictions in Chrome (https://bit.ly/trusted-types).

Remove the type attribute - `text/css` is default:
https://html.spec.whatwg.org/#update-a-style-block.

Closes #17014